Topic: High revving but throttle slides not sticking

[Ferk0417]

First post on this forum, been reading a lot and decided to join, seems like a nice community of people. I recently picked up a one owner 1974 cb750 with just over 20k on it. PO let it sit and carbs were all gummed up. I cleaned them out very very thoroughly and it started right up when they went back on. PO told me only problem he had was that sometimes the throttles seemed to stick , while carbs were off the slides moved freely and returned closed every time as they should. So I got it running and started To rev it, it would rev then drop normally for the first few twists of the throttle. Then it would seem to "stick" at 4k rpm and hang there for a while. The throttle returns fully, it is not sticky feeling, I can see all the push rods on the top of the carb go down fully and the bike continues to hang at 4k then will slowly drop down. I sprayed some carb cleaner all around intakes to check for air leaks and there seemed to be none at all. Boots are in good condition and clamps are all tight on carbs. So for a recap, I have cleaned carbs thoroughly, cleaned throttle and it moves and returns smoothly, brand new spark plugs, fresh gas. Don't know where else to go with this... Anyone have any ideas on what to check or try out? Anything you guys have will help no matter how simple it is I cannot think of anything else it could be. Thanks everyone

~ferk

[calj737]

There have been other threads citing similar conditions. Possible solutions are: advancer shaft is bent and the mechanism doesn't return from advance. Weak springs on the advancer, a sticky mechanism.

Remove the points cover, and use a timing light, rev it up and watch the timing plate to insure the timing is retarding normally.

[Ferk0417]

The shaft and advancer spring seems ok, like I said , I can see everything return as it should but it continues to rev. I have searched the forum for my same issue and didn't find any threads on it... Could you link me to the ones you have seen with similar problems?

[Ferk0417]

Also, it has electronic ignition

[edwardmorris]

When you cleaned them, did you take them apart fully then put them back together? If everything including the advancer is good, then you probably have your slide height set way too high. Try lowering it (evenly) to see if it helps. If you do this, or if you did take them apart fully, you WILL need to do a vacuum sync.

[edwardmorris]

Also, welcome!

[calj737]

Also, it has electronic ignition

Okay, but it still uses the stock mechanical advancer. The electronic ignition only replaces the points and condensers, but still relies on the plate being positioned relative to the advancer. So I'll encourage you to verify the timing returning with a light. "Looks okay" won't cut it when you're diagnosing problems. Use instruments to validate specs.

[Ferk0417]

When you cleaned them, did you take them apart fully then put them back together? If everything including the advancer is good, then you probably have your slide height set way too high. Try lowering it (evenly) to see if it helps. If you do this, or if you did take them apart fully, you WILL need to do a vacuum sync.

I did not take the whole throttle linkage apart. How do I lower slide heights? Is that with the idle screw? Also, is it possible I have some sort of vacuum leak even though when I sprayed carb cleaner around the intakes it did not rev up?

[edwardmorris]

Not the throttle linkage, the slide assembly for each carb. If that was taken apart (the large and small hex nuts disturbed) then the slide height may have changed.

[78whiteorbs]

If you place a hand over the intake of the carbs or choke it when it's hanging at 4k does it settle down? If yes possible air leak/ carbs not fully seated in boots.

If no it would seem to be mechanical problem . You can disconnect the cables and manually open the throttle with something and see if it hangs. This rules out a cable problem.

[Ferk0417]

If you place a hand over the intake of the carbs or choke it when it's hanging at 4k does it settle down? If yes possible air leak/ carbs not fully seated in boots.

If no it would seem to be mechanical problem . You can disconnect the cables and manually open the throttle with something and see if it hangs. This rules out a cable problem.

I will give that a try tmrw. Thank you

[Ferk0417]

Not the throttle linkage, the slide assembly for each carb. If that was taken apart (the large and small hex nuts disturbed) then the slide height may have changed.

Did not touch those but was thinking about adjusting them tmrw per my manual. I will give that a try as well

[ekpent]

Did you actually take the slides out when you did your cleaning on the gummed up carbs. I had a 74 with an idle issue and it was corrected when I removed the slides and cleaned them well and the bore the second time around. Also make sure the slide rods aren't binding on a bad rubber grommet on the carb top, little oil there is a good idea. While your fiddling around also try removing your return throttle cable,bottom one on carb, and test run to see if that could be an issue I have seen before.
   The idle increases on these bikes as they warm, make sure your idle is set with the large knurled knob correctly. As for the slides it just takes a very small amount of movement to drastically change the idle.

[Ferk0417]

Did you actually take the slides out when you did your cleaning on the gummed up carbs. I had a 74 with an idle issue and it was corrected when I removed the slides and cleaned them well and the bore the second time around. Also make sure the slide rods aren't binding on a bad rubber grommet on the carb top, little oil there is a good idea. While your fiddling around also try removing your return throttle cable,bottom one on carb, and test run to see if that could be an issue I have seen before.
   The idle increases on these bikes as they warm, make sure your idle is set with the large knurled knob correctly. As for the slides it just takes a very small amount of movement to drastically change the idle.

Rubber grommet was exactly the problem it was getting hung up in the throttle forks. It doesn't hang idle now just runs really lean and bogs and lacks a lot of power. Got the carbs off again to see what I messed up. Just got in my 4 into 1 Mac exhaust today. Might go one notch richer on all the needles